This invention relates to can bodies comprising a side wall provided with grooves; and furthermore to a method and apparatus for making the can bodies.
Ribs and panels have been formed in the side wall of cans for several purposes such as:
a). To give an interesting appearance to the can body which may promote a brand image. Such can bodies are shown in British Design Registration 1022336 and U.S. Design Pat. No. 290,688. In both cases the can body depicted is a drawn seamless can body having flange, neck and shoulder from which depends an array of longitudinal ribs and panels: the panels appear to be broad and thus substantially chordal in relation to the round body;
b). Longitudinal ribs and panels have been used to give rigidity to the side wall against loads applied to the top of the can body, such as arise during double seaming of a can end to close the body, or during stacking of filled cans. This aspect is discussed in WO 91/11275 where it is said that the panels between the ribs extend chordally to strengthen the side wall and so permit use of thinner side walls. However when a can, having flat or shallow externally concave panels in a thin side wall, is filled and closed with a beverage that generates pressure, the panels are pushed outwardly to make the original rib and panel shapes less visible. Whilst this problem may be overcome by forming deeper chordal panels, as shown in U.S. Design Pat. No. 332,750 there is a limit to the ductility of a side wall made of double reduced tinplate or wall ironed steel.
c). To provide flexible panels which move to accommodate volume changes arising therefrom during filling and thermal processing of the contents of the closed can, and settle for the final shape arising. Such cans are described in U.S. Pat. No. 5,040,698 and are particularly suitable for containing thermally processed foods. The same principle may be applied to a can for pressurised beverages if the width of the longitudinal panel is narrow e.g over 30 panels for a can body 65 mm diameter; however there persists the problem that the shallow concave panels may be forced back to a continuous cylinder if pressure within the can is high and ribbed cans with concave panels are susceptible to damage to their coatings during manufacture and abrasion during transport.
U.S. Pat. No. 4,953,738 (Stirbis) describes a one piece metallic can body comprising an end wall and a cylindrical relatively thin side wall provided with a first annular groove at a short distance from the open end of the side wall and a second annular groove at a short distance from the bottom wall. A number of longitudinal externally concave grooves extend between the annular grooves to define a like number of panels in between the longitudinal grooves. In FIG. 2 Stirbis shows that the panels are of externally arcuate cross section approximating to the original cylindrical side wall before grooving so that the grooves are formed of stretch-formed material. The side wall of a can body made by drawing and wall ironing tinplate or aluminium is work hardened and has as little as 5% elongation to fracture. In order to avoid risk of fracture this invention seeks to provide a form of grooves and panels, and means to make them, that do not subject the side wall to excessive stretching.
EP-A-0547636 describes a drawn and wall ironed can body which has alternating inward and outward segments around its circumference. The apparatus used to achieve this configuration uses a hard mandrel with inward and outward segments which carries the can body. The can body is forced to conform with these segments by compressing the can against a plate covered with a resilient layer.